Okay, now for some other thoughts I have (in an additional post so my other one isn't even longer

)
Cities
-Upgradeable buildings. Certain buildings should be able to be "upgraded" to other types, for additional bonuses.
-More buildings should go obsolete - you would lose the bonuses, but it would become a tourist attraction, therefore generating additional income for you.
-Once a city reaches a certain size, it could begin to "spill over" into surrounding terrain. These "suburbs" could produce additional income and production and could also allow for constructing additional types of certain improvements (but not all). Towns could also be absorbed by expanding cities.
-Ideas for new city improvements:
(some of these exist as unique buildings already, but I don't think they should be):
-Casino
-Dam
-Mall
-Museum
-Sewer system
-Ski resort
-Stadium
-Stock exchange
-Trading post
Land/Terrain
-NO MORE CITIES ON JUNGLE AND TUNDRA. Or at the very least, not until late in the game.
-Bridges (only over 1 tile of water)
-Canals. These could be built through single tiles of land between 2 oceans. The team that controls the canal could either have the option of closing it off, not allowing other teams to pass through it, or they could tax it and make major amounts of money (it would provide additional commerce just for existing, but you could also get revenue for each ship of another nation that passes through).
-Canals and bridges would both be available late in the game, perhaps with some new late-game technology (advanced engineering?).
-Multiple mountain levels. Low, high, and "peak". Peaks would be like the tallest, most monumental mountains on Earth. They would be impassible to everything. High mountains - you can't pass them until you build a road on them. Low mountains would be like mountains are now.
-Some terrain could generate tourist income. It would be randomly determined (and not unbalanced in anybody's favor), although certain terrain (such as forests or mountains) would be more likely to generate it. It would be a modest boost to income at first...more if you build a road to it. Certain advances and improvements later could generate even more income.
-National parks. Once you discover ecology (or its equivalent), you could designate certain areas national parks, generating even more revenue, but preventing any additional development from occurring.
-Elevation - have several different elevation levels (maybe 3 - then above that is hills, then mountains). Units defending at a higher elevation would get a slight bonus (perhaps 5%).
-Also, there could perhaps be "cliffs" between certain tiles, which would make it impossible to move onto that tile from that direction. Roads also wouldn't connect. Cliffs could also exist along the coastline, preventing naval landings. Cities on cliffs, however, wouldn't be able to build harbors.
-Canyons could also run between some tiles - they would be impassable until late in the game when you can build bridges across them.
-Cliffs and canyons may be too specific for some but I think it would add a little bit more strategy to the game without over-complicating it.
Diplomatic options:
-3 levels of border control (closed, open to non-combat units, and open to militaries). I know this has been discussed before in detail but it still makes sense.
-Diplomatic dialogue between multiple nations. Diplomacy often involves more than 2 powers. You could perhaps contact multiple nations to formulate a peace deal in a large war, for example, or in the reverse, to mutually declare war or enact trade embargoes. A civilization confronted with a demand from more than 1 nation at once would increase the chances of that nation obliging.
-There should be the ability for limited combat to occur without war occurring. Maybe a border skirmish could occur between 2 powers. It would increase tensions, and MAY lead to war, but wouldn't necessarily.
-United Nations peacekeeping missions. I've seen this discussed before, but I think it has some potential.
-I don't think every nation should have to join the U.N. Also, you should be able to violate U.N. resolutions, although it may result in expulsion from the U.N. and could also cause the member nations of the U.N. to unite against you.
Those are the major things I have for now, although more is likely to come.